linux-kernel - Re: [PATCH v2 3/3] netfilter: x_tables: Use correct memory barriers.

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Message-ID: <20210309113348.GE10808@breakpoint.cc>
Date:   Tue, 9 Mar 2021 12:33:48 +0100
From:   Florian Westphal <fw@...len.de>
To:     Mark Tomlinson <mark.tomlinson@...iedtelesis.co.nz>
Cc:     pablo@...filter.org, kadlec@...filter.org, fw@...len.de,
        subashab@...eaurora.org, netfilter-devel@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/3] netfilter: x_tables: Use correct memory barriers.

Mark Tomlinson <mark.tomlinson@...iedtelesis.co.nz> wrote:
> When a new table value was assigned, it was followed by a write memory
> barrier. This ensured that all writes before this point would complete
> before any writes after this point. However, to determine whether the
> rules are unused, the sequence counter is read. To ensure that all
> writes have been done before these reads, a full memory barrier is
> needed, not just a write memory barrier. The same argument applies when
> incrementing the counter, before the rules are read.
> 
> Changing to using smp_mb() instead of smp_wmb() fixes the kernel panic
> reported in cc00bcaa5899 (which is still present), while still
> maintaining the same speed of replacing tables.
> 
> The smb_mb() barriers potentially slow the packet path, however testing
> has shown no measurable change in performance on a 4-core MIPS64
> platform.

Okay, thanks for testing.  I have no further feedback.